[ruby-core:104604] [Ruby master Bug#17666] Thread#join hangs when Fiber.set_scheduler is set

Issue #17666 has been updated by ioquatix (Samuel Williams).


I think we should consider isolating this for backport to 3.0.3 if possible.

This three lines:

https://github.com/ruby/ruby/blob/v3_0_2/thread.c#L547-L549

```c
        if (target_thread->scheduler != Qnil) {
            rb_scheduler_unblock(target_thread->scheduler, target_thread->self, rb_fiberptr_self(join_list->fiber));
        } else {
```

should become:

```c
        if (target_thread->scheduler != Qnil && rb_fiberptr_blocking(join_list->fiber) == 0) {
            rb_fiber_scheduler_unblock(target_thread->scheduler, target_thread->self, rb_fiberptr_self(join_list->fiber));
        }
```

We would need to backport `rb_fiberptr_blocking` from `cont.c` too:

```c
unsigned int rb_fiberptr_blocking(struct rb_fiber_struct *fiber)
{
    return fiber->blocking;
}
```

There are some other usage in `thread_sync.c` which might be candidate for backport.

cc @nagachika

----------------------------------------
Bug #17666: Thread#join hangs when Fiber.set_scheduler is set
https://bugs.ruby-lang.org/issues/17666#change-92891

* Author: arjunmdas (arjun das)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------

``` ruby
class MockScheduler
  def block(blocker, timeout = nil)
    byebug
  end

  def close
    byebug
  end

  def fiber(&block)
    byebug
    Fiber.new(blocking: false, &block).tap(&:resume)
  end

  def io_wait(io, events, timeout)
    byebug
  end

  def kernel_sleep(duration=nil)
    byebug
    Fiber.yield
  end

  def process_wait(pid, flags)
    byebug
  end

  def unblock(blocker, fiber)
    byebug
  end
end
  
Fiber.set_scheduler(MockScheduler.new)

t1 = Thread.new do
       p 'before'
       sleep 1
       p 'after'
     end

t1.join
```

Code hangs at this point.




-- 
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